This is a common misconception. Topical tretinoin has been shown to have no effect on the fetus. Oral tretinoins (like isotretinoin) have been linked to congenital defects. This is due to the differing rates of absorption and the resulting ability for the retinoid to cross the placenta at higher concentrations.

With that said, folks who are pregnant are discouraged from using any retinoids out of an abundance of caution.